In integrated circuit technology, devices in different integrated circuit (hereinafter referred to as "IC") packages are interconnected to one another at I/O PADS associated with each IC package. I/O PADS are associated with electrical circuits which perform a desired function to interface with other IC packages or electrical devices. An I/O PAD may be associated with electrical circuits which generate output signals and apply the signals to the I/O PAD for external devices to sense and process accordingly. Alternatively, an I/O PAD may be associated with electrical circuits which sense the logic state of signals applied to the I/O PAD by external electrical circuits or IC packages. I/O PADS are frequently "bi-directional" in the sense that they may be used at different times for the sensing of input signals to the IC package or for the application of output signals from the IC package. Electrical output signals are applied to an I/O PAD by electrical circuits within the IC package associated with the I/O PAD. Similarly, electrical input signals are received as input signals from an I/O PAD by associated electrical circuits within the IC package which "sense" the signal level and operate accordingly.
It is common for such interconnected circuits to utilize standard voltage levels to represent logic states of 0 and 1 (or "on" and "off"). Common standard voltage levels in the past have been 0 Volts (.+-. a threshold value) to represent one logic state and 3 Volts (more precisely, 3.3 Volts .+-. a threshold value) to represent the other logic state. As new IC manufacturing technologies evolve, the voltage levels used may change. For example, in the manufacture of many current IC devices using submicron semiconductor fabrication processes, the semiconductor industry has begun to standardize on 2.5 Volt (.+-. a threshold value) in place of 3 Volt signal levels to improve performance and reduce power dissipation. The lower voltage level permits reduced thickness in transistor gate oxide materials to thereby reduce switching time of transistor gates and improve performance of the switching circuitry. As a result, input/output circuits fabricated in a 2.5 Volt CMOS technology must include circuitry for interfacing to a 3.3 Volt circuit by shifting 2.5 Volt signals to 3.3 Volt signals or, in other words, transforming a signal ranging from 0 to 2.5 Volts into a signal which ranges from 0 to 3.3 Volts. This transformation of the signal is referred to as "voltage level shifting." Due to the bidirectional nature of input/output circuits, these circuits must also include a tri-state function which enables the output of a driver for outputting information from the IC package and disables the output of the driver by putting the output of the driver in a high impedance mode when information is being inputted into the integrated circuit package.
Drivers in such input/output circuits must also include slew rate control for reducing noise. Existing driver circuits for input/output circuits have separate functions for performing voltage level shifting slew rate control or tri-state capability or some combination of the three functions. By putting these functions into multiple stages, significant delay is introduced into the I/O circuit.
What is needed is an input/output circuit including a pre-drive circuit which incorporates all three functions, i.e., the level shifting function, slew rate control function, and the tri-state function in a single circuit to avoid delay introduced by putting those functions into multiple stages.